Update actions/checkout action to v4 (#25)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/main.yml

@@ -18,7 +18,7 @@ jobs:
           - 3.1.4
           - 3.2.2
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}